Probe Data Sampling Guidelines for Characterizing Arterial Travel Time

Probe data are emerging as an important source for characterizing transportation systems. Travel time distributions have traditionally been characterized by the mean and standard deviation. These statistics work well to characterize uncongested freeway systems, which have travel time distributions that are approximately normal. When congested conditions or interrupted-flow facilities are encountered, the travel time distributions become more complex. Recently some additional travel time reliability indexes have been developed to quantify these travel time distribution characteristics. This study develops mathematical techniques for determining the sample size required for estimating the underlying travel time distributions that can be used for assessing changes in travel time distributions associated with operational changes of traffic signal controller offsets. The example provided shows that while gross changes in offsets require approximately 7 probe vehicle samples per study interval, subtle changes in offsets require approximately 80 probe vehicle data samples per study interval. Although these guidelines were developed for evaluating offset changes, the mathematical framework can be applied for evaluating the impact of other parameters, such as split times and cycle lengths. Further research on applying these mathematical techniques to a broader cross section of traffic conditions is warranted to assess their transferability to oversaturated conditions and freeways.